Superconducting air-core, synchronous electric machines have been under development since the early 1960's. The use of superconducting windings in these machines has resulted in a significant increase in the field electromotive forces generated by the windings and increased flux and power densities of the machines.
Early superconducting machines included field windings wound with low temperature superconductor (LTS) materials, such as NbZr or NbTi and later with Nb3Sn. The field windings were cooled with liquid helium from a stationary liquifier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both the latent and sensible heat of the fluid to cool the windings. This approach proved to be viable for only very large synchronous machines. With the advent of high temperature superconductor (HTS) materials in the 1980's, the cooling requirements of these machines were greatly reduced and smaller superconducting machines were realizable.
In superconducting machinery, efficiency and size are of critical importance. One way of reducing the size of a superconducting machine is to minimize the air gap between the field windings and the stator windings. Superconducting rotor windings typically utilize some form of metallic shielding between these windings to minimize the detrimental affect of asynchronous fields from the stator windings. Unfortunately, during hi-load use and various fault conditions, this metallic shield will be exposed to considerable amounts of thermal loading. This thermal loading can be problematic, as the shield (which is typically constructed of aluminum and rigidly attached to the axial ends of the rotor) will want to expand along the axis of the rotor.